This invention relates to television pick-up tubes and particularly to an improved mesh assembly having reduced microphonics for such a tube.
Microphonics is a physical vibration of some portion of a tube which, because of its movement, produces an undesirable electrical signal output or noise which manifests itself as a background of lines or striations in a television picture.
In a vidicon type pick-up tube, a mesh grid is disposed between a photoconductive target and an electron gun which provides a scanning electron beam. The mesh provides a lens action which causes the electron beam from the electron gun to impinge perpendicularly on the target electrode. The mesh is usually supported around its periphery by at least one annular support ring.
When such tubes are subjected to mechanical shock, for example in vehicles or equipment, and vibration, for example, from cooling fans or incident sound from external sources, the conductive mesh will start vibrating relative to the photoconductive target and will cause microphonics effects which produce the above-described background lines and striations in the picture.
Many expedients such as rectangularly shaped damping members in contact with the mesh have been adopted to eliminate the undesirable microphonic effects. Such configurations provide a rectangularly shaped raster format and thus limit the orientation of the tube within a multitude color camera thereby complicating the tube alignment procedure. Among the other expedients adopted is a structure described in U.S. Pat. No. 3,906,278 to Horton et. al., issued Sept. 16, 1975 comprising a fine flexible mesh washer clamped between two annular members. The annular members are dished away from each other beyond the region of clamping. The flexible mesh washer extends inwardly beyond the region of clamping and contacts both the fine conductive mesh and the dished portion of one of the clamping members in order to damp vibrations. The Horton et al. structure is complex and requires a precisely formed flexible mesh washer. If the corrugations of the mesh washer are too shallow the washer will not contact both the dished annular member and the fine mesh. If this occurs, little or no damping will occur. Being constructed from mesh, the flexible washer also tends to undergo a change in elasticity after repeated thermal and mechanical cycling thus decreasing the effectiveness of the damping action. It is therefore desirable to find a low cost, reliable mesh mounting system which provides sufficient mesh tautness and damping to reduce microphonics.
One such mesh mounting system is described in copending U.S. patent application, Ser. No. 150,341, filed on May 16, 1980 by Benner et al., and entitled, "Mesh Assembly Having Reduced Microphonics For a Pick-Up Tube". The Benner et al. application is assigned to the same assignee as the present invention and is incorporated by reference herein for disclosure purposes.
The Benner et al. structure comprises a mesh electrode disposed between a frustro-conically shaped mesh support ring and a dished, i.e. frustro-conically shaped, spring-like mesh damping ring. The mesh damping ring is compressed into a reversal of its dished shaped and welded at a plurality of points to the outer periphery of the mesh support ring. By reversing the dished shaped of the mesh damping ring, the damping ring assumes an undulatory or serpentine configuration which contacts the mesh electrode periodically around the inner periphery of the damping ring. The area of contact extends radially outward from the inner periphery to the weld points at the outer periphery of the damping ring.
The aforedescribed Benner et al. structure is presently used in the RCA Vistacon, a trade name for a lead-monoxide vidicon. Vistacons using the Benner et al. mesh mounting system have excellent damping time of about 1 second or less. However, the Benner et al. structure requires accurate control of the temper of the mesh damping ring as well as proper weld placement at the periphery of the ring to assure that the damping ring assumes the proper undulatory configuration necessary to properly damp the mesh vibration. Improper damping ring temper or welding procedures reduce the number of periodically located contact points between the damping ring and the mesh and thereby decrease the damping efficiency of the mesh mounting system.